The present invention relates to lasers and particularly to a tunable solid state laser for producing a laser emission at one or more laser wavelengths simultaneously over a preselected wavelength range.
Multifrequency operation of a tunable solid state laser is desirable when there is independent wavelengths and bandwidth control of each output wavelength since such a device can be useful for such applications as differential absorption LIDAR (DIAL) and sum frequency generation. Early demonstrations of multifrequency devices concentrated on pulsed dye lasers, see for example H. S. Pilloff, "Simultaneous Two-Wavelength Selection in the N.sub.2 Laser Pumped Dye Laser," Applied Physics Letters, Vol. 21, pp. 339-340, 1972; C. Wu and J. R. Lombardi, "Simultaneous Two-Frequency Oscillation in a Dye Laser System," Optics Communications, vol. 7, pp. 233-236, 1973; H. Lotem and R. T. Lynch, Jr., "Double Wavelength Laser," Applied Physics Letters, Vol. 27, pp. 344-346, 1975. The technique of using a dye laser in a pulse mode for simultaneous operation is both cumbersome and complicated and gives rise to numerous problems. For example, the gain medium lifetime is fairly short, requiring frequent changes of dye and solvent. Dye solvents are often flammable and dyes themselves are generally carcinogenic. In addition, the techniques used to generate simultaneous multifrequency operation in the pulse dye lasers use inefficient means to separate and tune the wavelengths and these techniques therefore are not suitable for CW operation. The titanium-doped sapphire (Ti:sapphire) laser is a tunable laser that operates over the wavelength range of about 680 nanometers to about 1.1 microns and is ideally suited for a number of applications that previously had used dye lasers. The Ti:sapphire gain material is presently grown in very high quality and it has been shown to be an extremely efficient laser material. However, prior operation of the Ti:sapphire laser had been at only one frequency (or wavelength) at a time. Simultaneous multifrequency operation had not been demonstrated previously in the Ti:sapphire laser, and cw multifrequency operation had not been previously demonstrated in any tunable laser. It is to be noted that the terms "wavelength" and "frequency" as used in this patent are interchangeable. The relationship between the frequency v and the wavelength .lambda. of
optical radiation is given by the well-known equation .lambda.v=c, where c is the speed of light.
A U.S. Pat. No. 4,287,486, entitled "Laser Resonator Cavities with Wavelength Tuning Arrangements", by Ali Javan, discusses various means of using tunable gain media to obtain multifrequency operation where all wavelengths can emerge collinearly. Independent control of wavelength and spatial separation between wavelengths is provided. Javan's design is most appropriate for lasers that are either discharge pumped such as gas lasers or optically pumped from the side. The unique features of end pumping are not taken advantage of nor are they addressed in Javan's patent. In addition, although Javan shows an intracavity prism as a wavelength dispersive element, no attempts are made to collimate the beam through the prism in the laser resonator cavity and dispersion by that prism is therefore degraded. An apparent limitation in Javan's patent is that wavelength tuning is accomplished by use of a small aperture which may lead to unacceptably high diffraction losses for a cw pumped laser. If the aperture is made wider to reduce these losses, the laser bandwidth would increase and wavelength selectivity would be degraded. An additional apparent limitation is that the Javan laser does not operate naturally in the lowest order transverse mode so that a hard aperture must be inserted in the laser resonator cavity to achieve the low order spatial mode operation; but this aperture also would increase the passive loss of the laser resonator cavity to unacceptably higher levels for cw operation.
Thus, a continuing need exists in the state of the art for an operational cw multifrequency solid state laser having the laser resonator cavity mode focused at the gain element thereby providing for very high optical pump efficiency that may be efficiently end-pumped, is practical, contains a region where the laser resonator cavity mode is collimated thereby allowing optimal use of an intracavity prism used in a minimum deviation orientation, as well as allowing for insertion of birefringent wavelength tuning elements and intracavity etalons for line narrowed operation, and is readily tuned such as by independent wavelength control at each wavelength by angular tuning of one mirror or by rotating the prism to permit the simultaneous scanning of all wavelengths while the separation between the wavelengths is maintained constant when pumped with suitable optical means.